Microwave and millimeter wave circuits are extensively employed in, for example, high-frequency satellite communication and radar systems. Receive networks in these and other high-frequency systems generally include sensitive microwave components, such as mixers and low-noise amplifiers, for detecting transmitted microwave or millimeter wave signals. In order to protect these components from high-power signals (e.g., radar jamming signals) incident on a receive antenna, power-limiting circuits are often interposed in series between the antenna and the receiver. Microwave and millimeter wave limiters are designed to present a characteristic transmission line impedance with minimal insertion loss when signals of ordinary magnitude are being received by the antenna. In contrast, high-power signals impinging on the antenna cause the limiter to approximate a short circuit, thereby preventing damage to sensitive components within the receiver.
In high-frequency systems having an array of antenna input ports, switches are used to select or distribute microwave signals. Microwave switches may be employed to, for example, couple a main signal path or "common arm" to the limiter circuit associated with a particular antenna. Each antenna input port is typically followed by the cascade connection of a limiter circuit and a switching element. The term switch-limiter is often used to describe such a cascade connection.
FIG. 1 is a schematic representation of a conventional limiter circuit driven by a Schottky diode D1 positioned proximate a high-power input port. The input port will generally be in communication with an antenna, and also serves as the origin of a high-frequency signal path P. The signal path P will typically be realized as microstrip or stripline, with the diode D1 and a bypass capacitor C2 being coupled to the signal path P by a quarter-wave coupler Q. The position of the quarter-wave coupler Q relative to the signal path P may be varied so as to adjust the coupling between the diode D1 and the signal path P. Since the diode D1 is turned on in response to high-frequency signal energy of known power, the coupler Q may be positioned such that the diode D1 is actuated when signal energy in excess of a predefined power impinges upon the input port. In this way the limiter may be calibrated so as to present a short-circuit to signal energy of predetermined magnitude received by the input port.
More specifically, when a high-power pulse exceeding the requisite magnitude appears at the input port the diode D1 is turned on by high-frequency signal energy supplied thereto by the coupler Q. Upon being turned on the diode D1 generates a DC bias current, which is supplied to PIN diodes D3, D4 and D5 through a bias structure. The bias structure includes resistor R1, RF choke inductor L1 and capacitor C3. Since the bias current turns on diodes D3, D4 and D5, several high-frequency short circuits are created between the signal path P and ground. In this way the high-power pulse is reflected first by the short circuits at parallel-connected PIN diodes D3 and D4, while diode D5 reflects pulse energy able to bypass diodes D3 and D4 along signal path P. Attenuation of high-power signals between the input port and the protected output port will generally be maximized by spacing diode D5 from diodes D3 and D4 by a quarter-wavelength of the center frequency transmitted by the path P.
As is evident from FIG. 1, the bias current supplied by diode D1 to diodes D3, D4 and D5 when a high-power pulse is incident upon the input port depends upon the magnitude of R1. Large values of R1 allow a relatively greater percentage of the bias current from the Schottky diode D1 to be supplied to the diodes D3, D4, and D5, while smaller values of R1 result in increased dissipation of bias current. Accordingly, power-handling capability is improved by augmenting the resistance R1 such that the bias current generated by diode D1 is predominantly used to energize the diodes D3, D4 and D5.
The advantages of Schottky driven limiters such as that shown in FIG. 1 include high power-handling capability (e.g., 2 kW for a 1 .mu.s pulse or 50 W continuous-wave), an adjustable limiting threshold provided by coupler Q, and the absence of a requirement that the parameters of the PIN diodes D3, D4 and D5 be matched.
Unfortunately, Schottky limiters are known to have relatively long recovery times, where recovery time is defined as the time required for the limiter to return to a low-loss state subsequent to the end of a high-power pulse. Upon cessation of the high-power pulse, diode D1 turns off and terminates the supply of bias current to diodes D3, D4 and D5. This allows the PIN diodes D3, D4 and D5 to return to a non-conducting (i.e., "insertion-loss") state after residual charge stored in the intrinsic regions thereof is removed. While in the insertion-loss state the diodes D3, D4, and D5 appear essentially as open-circuits to signal energy propagating along the path P, and hence allow low-power signals to pass between the input and protected ports. However, diodes D3, D4 and D5 behave as open circuits only after the residual charge stored within each is completely removed subsequent to the diminution, or "limiting", of a high-power pulse. It follows that the diodes D3, D4 and D5 undesirably reflect a portion of low-power signal energy for a characteristic "recovery" time subsequent to incidence of a high-power pulse, thereby undesirably increasing overall insertion loss (i.e., signal attenuation).
As may be appreciated by referring to FIG. 1, the recovery time required for the diodes D3, D4 and D5 to return to the insertion loss state following incidence of a high-power pulse is inversely proportional to the magnitude of R1. Hence, small values of R1 shorten recovery time, while large values of R1 increase recovery time. However, small values of R1 result in larger fractions of the bias current from diode D1 being diverted to ground, thereby reducing the power-handling capability of the PIN diodes D3, D4 and D5. As a consequence, designers of conventional limiters are typically forced to strike a compromise between power-handling and recovery time when selecting the resistance value of R1.
As noted above, the conventional limiter of FIG. 1 is typically cascaded in series with a high-frequency switching circuit in order to form a switch-limiter circuit. Since switching circuits also generally include a pair of quarter-wave spaced diodes, such composite switch-limiter circuits will often incorporate at least four PIN diodes connected in shunt with the high-frequency signal path. Unfortunately, this redundancy undesirably increases the insertion loss of conventional switch-limiter circuits, which in turn tends to limit the range of the antenna system in which the switch-limiter is disposed.